You're not!
Part of any child's development is to learn where the barriers are, and then push them. You respond with firm but fair discimpline and she will respond.
If you have said 'No' firmly, and she kicks off, walk away. Tantrums are designed to get attention - remove the attention, the tantrum stops. Before long, your little angel will learn that if she throws shapes in supermarkets, people will look, and you will be embarassed. Same response, trun your back on her until she stops, which won't be long.
In time she will learn that screaming fits are not only tirin,g they don;t work, and she ill stop them, you just have to be patient and ride this phase out.
All mums worry that they are getting it right - no-one does get it right all the time, but you will learn as you go, and so will your daughter.
Remember, boundaries are there to be laid down, and adhered to, that way lies a secure child, who will push you, but you have to make sure you always win.